1. Field of the Invention
The present invention relates to a method of a radar detecting a target by emitting a radio wave and by receiving its reflection wave, and more particularly, to a pulse radar apparatus which emits a transmission radio wave of a high frequency in a pulse form, by generally, separating it into equal sections and by putting them, is available for a short-distance measurement, and has a high resolution.
2. Description of the Related Art
For currently used radars, most of them are pulse radars. The pulse radars can generally detect a target that exists in a long distance, and can measure a distance to the target. Various signal processing techniques used for such pulse radars are described in “M. Sekine, Radar Signal Processing Techniques, The Institute of Electronics, Information and Communication Engineers”.
Additionally, as conventional techniques for detecting a target that exists in a relatively short distance, the following techniques exist. Firstly, “Moriue, Nakatsukasa, A Method for Distance/Speed Measurement of Short-distance Mobile Object, National Convention of IEICE, 2000, B-2-2, p. 215”proposes a method measuring the distance and the speed of a mobile object in a distance of up to 125 m by using a microwave of a 9.5-GHz band, which is amplitude-modulated by a sinusoidal wave signal.
Additionally, Japanese Patent Publication No. 2001-116822 “Microwave Band Pulse transmitter/Receiver” discloses a microwave band pulse transmitter/receiver of a small size, low-cost, and low power consumption, which uses a microwave band weak radio wave and is fit for uses such as a data communication, a sensor, a measuring instrument, etc. Also in this document, a gate is used in a similar manner as in embodiment of the present invention. The gate in this document, however, is intended to suppress oscillation, and it use purpose is different from that of the present invention.
Furthermore, Japanese Patent Publication No. 2000-241535 “Short-Distance Radar Apparatus” discloses a high-resolution and short-distance radar the use of which is permitted by a simple license application, and the outdoor use of which is enabled without worrying about a radio wave interference, and various applications of which are expected to be implemented with an outdoor non-contact distance measurement.
As described above, the conventional pulse radars were used in a relatively long distance to a target, which is equal to or longer than several tens of meters. To use a pulse radar for a short-distance measurement, its pulses must be sharpened, the frequency bandwidth for the application must be widened, and the bandwidth of a component to be used for the radar must be widened, so that its implementation is difficult.
FIGS. 1 and 2 explain the bandwidths of this pulse radar. FIG. 1 explains the bandwidths used for normal AM and FM signals, etc. For the AM and the FM signals, their bandwidths are limited to narrow ones that center the frequency of a carrier wave. Therefore, a noise influence can be suppressed.
FIG. 2 explains the bandwidth of a pulse radar. The narrower the pulses of a pulse radar, the wider its bandwidth. As a result, noise power N of a total bandwidth becomes large even if signal power S is the same, so that an S/N (signal-to-noise) ratio is deteriorated, and the radar becomes susceptible to a noise influence. Especially, the S/N ratio is deteriorated at 1 GHz or higher, and various problems can occur. If the width of a pulse is widened to suppress the S/N ratio, the bandwidth becomes narrow and the noise N becomes small. However, the minimum distance to a detectable target becomes long.
To set the short-distance detection limit to on the order of 15 cm with the above described conventional techniques of pulse radars, the width of a pulse must be changed to on the order of 1 nsec. To implement this, a bandwidth of approximately 1 GHz is required, and also a noise bandwidth is as wide as 1 GHz, so that the S/N ratio is significantly deteriorated, and it becomes very difficult to detect a target.
Additionally, a general-purpose digital LSI such as a DSP, etc. cannot be used, because a signal having a pulse width of approximately 1 nsec or a frequency bandwidth of approximately 1 GHz is handled. Therefore, a circuit must be configured with a semiconductor specifically developed for high-speed processing, which costs very high, and a mass-production is difficult due to fluctuations of characteristics.
Additionally, the conventional techniques generally use a method performing the I-Q detection for a reflection wave, namely, a reception wave, obtaining an I (synchronous) component and a Q (orthogonal) component with a reference sinusoidal wave, performing A/D conversion respectively for the I and the Q components, and executing signal processing with a processor for its result. However, this method requires A/D converters and filters for two systems, leading to an increase in cost.